Quantum entanglement

Quantum entanglement is an area of science that will one day dominate the way we look at information, the way we communicate secretly and the way our computers do their thing. Problem is, it's really bloody confusing!

By Duncan McKimm

Classical computers use binary code to store as much information as possible. (Source: Reuters)

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At the moment, quantum mechanics is one of those fields of science that gets scientists really excited, but that not many people outside of science have ever heard about. Kind of like Star Trek. Despite this, quantum entanglement is a field that is going to change the way we do almost everything, from the way our computers function, to the way we think about information itself (unlike Star Trek).

Quantum computer chips will allow our mobile phones and PC's to keep getting tinier and tinier, something that our current technology can't to do for very much longer. We are nearly at the top of the quantum roller coaster's climb — it's still pretty hard going to make a step forward — but every advance in quantum technology brings us closer to that peak. Once we get there, there won't be any stopping quantum technology, and in no time it will be everywhere.

Quantum cryptography will make sure that we can send our messages in total security, by changing the way we create codes, to make them virtually unbreakable.

These developments hinge on quantum mechanics, which comes down to how we think about information - and when it comes to quantum, everything changes...

The weird world of quantum entanglement

Quantum entanglement is a vital part of some of the new quantum technology, but unfortunately it is shrouded in mystery, partly because entanglement is inherently strange, but also because it can be very confusing to understand. Quantum entanglement is something that even Einstein had a little difficulty with, so it's certainly no walk in the scientific park. To understand quantum entanglement though, a bit of background on quantum mechanics goes a long way.

Our world is made up of tiny little bits of matter — let's call it "stuff". Even light is made of "stuff", called photons. These are particles of light — seems simple enough, but light is a bit trickier than that.

The tricky thing about light is that it can be both a wave and a particle at the same time. In what's called the "double slit" experiment, physicists can show light behaving as a particle and a wave seemingly simultaneously. This seems impossible, but despite offending our sensibilities, light goes on in this 'dual state' without a care.

A. Light is shone at a tiny hole in a screen, a hole that's smaller than the wavelength of the light, so that on the other side of the screen, the light emerges and spreads out, like ripples on a pond after a rock is thrown in.

B. This light then hits a second screen, which has two tiny holes in it.

C. When the light emerges from the second screen, it looks like two stones have been dropped into a pond, as the ripples of light spread out and overlap.

D. As the waves overlap, they interfere with each other — if a wave trough from one wave overlaps with a wave peak of another wave, it cancels out, so if there is a screen in the area after the light goes through the two holes, a dark spot will show up. If two peaks overlap, there'll be a light patch on the last screen and so on.

If you think of a wave trough like a $50 debt in your bank balance, and a wave peak like $50 of pay getting deposited — one week you might have a debt and then get paid, a peak overlapping with a trough, so your balance will be zero — but if a peak hits overlaps with another peak, another $50, you get the sum of both, $100. This is called destructive and constructive interference.

Getting weirder

The plate shows each photon as a dot of light as it hits, having passed through the slits. This shouldn't present significant problems, but if the experiment is slowed down, to only allow one photon at a time pass through the slits, we start to see some weird stuff. What you would expect to see is each photon pass through one of the two holes and hit the photographic screen directly behind that hole. After the experiment goes for a while, you would expect to see two bright patches behind each of the holes where the photons had passed through.

But as the experiment progresses and millions of photons are passed through the slits, the dots on the screen actually form the interference pattern that shows up when light behaves as a wave! But light should be behaving as a particle, because we are measuring it using the photographic plate, which should only show light in its particle state.

What is really weird is that this means because each photon is going through the experiment on its own, it must somehow travel through both the holes at once, and interfere with itself, before it lands on the screen in one of the bright spots of the interference bands that were seen in the first part of the experiment! Physicists say it is like the photon "knows" where it needs to end up, so it can form the interference pattern. So light is behaving as a particle and a wave at the same time. As one physicist, Ralph Baierlein put it, "light travels as a wave, but departs and arrives like a particle." This experiment has also been done with particles other than light, so it isn't just light being difficult, it's a case of some seriously weird quantum goings on.

The quantum level

When you get down to the level of photons and other really tiny particles, the rules that govern the world we live in, the "classical" world, give way to the rules of "quantum mechanics". As we saw with the double slit experiment, the light didn't behave as we expected it to, not because it is trying to confuse us (but it's doing a pretty good job anyway), but because it's playing by quantum, not classical rules.

Somewhere along the line, as things get smaller and smaller, the rules of the classical world change over to the rules of quantum. Physicists would expect that because in the double slit experiment the measurements are being taken by instruments that measure light in each state — particle and wave - that they would see light behaving as either one of those states, depending on which instrument, but not both.

Light is actually in a superposition, something else that is a huge part of quantum mechanics, although it is also seen in the classical world.

Understanding superpositions

"A superposition is two possible measurement outcomes with some coherent relationship between them," says Dr Andrew White, a physicist at the University of Queensland who's looking at the use of quantum entanglement in quantum computers.

Superposition is something that forms a very large part of understanding entanglement. As Andrew explained, superposition is actually very common. The light waves in the double slit experiment were in a physical superposition when they overlapped to create the bright bands on the screen (if you think of it in terms of the banking analogy we used before, you could say it was either $50 + $50 or $100).

Measuring destroys superpositions

If I flip a coin in a darkened room, the result of the coin being flipped is mathematically just as likely to be heads or tails. While the light is off, the coin is in a superposition — whereby it is both heads and tails at once, because I can't see which it is. If I turn on the light, I "collapse" the superposition, and force the coin to be either heads or tails by measuring it. Measuring something destroys the superposition, forcing it into being in just one classical state.

On a quantum level, you can see measurement is very destructive — even if something isn't in a superposition, just measuring a quantum object changes something, somewhere in the quantum system. That is why physicists expect to see light as either a particle or a wave, depending on whether they put the screen or the photo plate in the double slit experiment — because they're collapsing the superposition of the light, from being particle and wave at once to being either one or the other.

Quantum cryptography uses this destructiveness, because any message that is intercepted will be changed through being measured. Knowing someone is reading your messages means that you can only send information when you know it is not being intercepted.

Disentangling entanglement

"What entanglement is, is it's a superposition, of superpositions," he laughs,

"So instead of being two possible states, it's a superposition of two possible processes... and that's it!"

So entanglement is just a superposition of superpositions. Well, Andrew's explanation was good, but it is difficult to think about it in terms of particles. When particles are replaced with something that is more common it's a little easier to deal with.

Imagine a hypothetical TV show called "Ozzie Idle", where people sing pop music every week and get judged by an audience (bizarre I know, but go with it). Every week one person gets eliminated from the show, whoever gets the least amount of votes. The show is not very popular, and only three people, you and your two good friends, tune in every week, voting for who you think is the best. In the final week, the two last contestants perform, but your television breaks, so you can't watch or vote, leaving only your two friends able to vote. Your friends always vote for the same person, so in the final week you know that whoever they vote for will be the one that wins the competition.

You sit at home not being able to watch the show and unable to know the result until the next day. When you wake up in the morning, you rush out and buy a newspaper, so that you can see who won the competition. Before you find out the result, the situation is in a superposition. As soon as you pick up a paper, you will find out who won — and will know how your friends voted. Alternatively, if you talk to your friends and ask them who they voted for, you will know who won. But before you do either of these, there is a superposition where both final contestants won the competition, and where your friends voted for both contestants.

The reason it is entangled is that you can't know about one without it affecting the other. For example, you cannot look at the result in the paper without knowing who your friends voted for. Also, you can't talk to your friends without affecting the superposition of who won. This is the weird bit of entanglement, but it is what makes it so interesting and useful.

People like Andrew, that are making quantum computer parts, can take advantage of this weird connection to store and process more information. The more information you can process and store, the faster your computing system.

Many superpositions = a new way of computing

At the moment, computers use a binary code, 1's and 0's — called the computer's "language". The reason it uses only two "letters" is that in a classical computer, there is no advantage to having more letters, you can store more information, but it takes a heap longer to process, giving no real advantage in the wash-up. Quantum computers however, can use more than two letters, which gives them an advantage. They can get an advantage because they treat information in a different way, and process it differently, using entanglement of pairs of photons. Where a classical computer part can only give a yes or no answer to a specific problem, a quantum part can answer several questions in one calculation, because you can have answers that intertwine, like the Ozzie Idle results.

When particles are entangled, it gives them a unique relationship in terms of information, which means they can be very useful indeed for processing.

Quantum cryptography can use this as well, because the more ways there are to store information in a message, the harder it is to crack and when we're talking information in a quantum system, it begins to look almost unbreakable.

As computers get smaller and smaller, eventually they'll cross the border into the quantum world. When this happens, we'll have to play by quantum rules, so Andrew says 'Why not design a system that uses those rules anyway?' Quantum computing is going to be the future of the computing world, because it solves the problem of size and gives a power boost as well.

It's evolution, baby

So in the end quantum entanglement turned out to be quite strange, but not totally baffling. But why is it so hard to get a handle on quantum entanglement off the bat? Andrew says it's not because it's really difficult, but because we aren't built to think about things like particles (judging by your headache, I'm sure you won't argue).

"It's not because it's intrinsically hard, but because humans evolved to tell other humans there was good fruit and nice water, not to study the universe," he says.

I made it through in the end, and I don't feel too bad about not getting it straight away, but I think I'll leave the quantum physics to those more "evolved" characters in the lab.